There are many integrated circuits manufactured with programmable devices or features; that is, the user may alter the electrical characteristics of the integrated circuit to satisfy the user's needs. The features may be such that they may be altered many times; that is, they may be programmable, or they may be alterable only a single time. Programmable devices offer more versatility, but typically lose their programming information if the applied voltage is removed. Single time programmable devices retain information when the applied voltage is removed but are typically programmed in a destructive manner. They thus retain the programmed information, but, if programmed with the incorrect information, may become useless.
The programming may change a device from a low resistance state to a high resistance state or it may change a device from a high resistance state to a low resistance state. The former device is the well known fuse, and the latter device is now termed an antifuse. A typical antifuse, in the unprogrammed state, has a non-conducting region of, for example, amorphous silicon between two conducting regions. The amorphous silicon has a very high resistance and there is minimal conduction between the two conducting regions. When a sufficiently high voltage, termed a programming voltage, is applied between the two conducting regions, the amorphous silicon changes to a low resistance state. The conducting regions may be aluminum and there may be a barrier layer of, for example, TiN, between either or both of the aluminum regions and the amorphous silicon. There may also be a metal layer of, for example, Ti between the TiN and the amorphous silicon. The programming voltage creates a conducting path through the amorphous silicon. The programming voltage is proportioned to the thickness of the amorphous silicon since breakdown occurs at a specific electric field. The thickness of the amorphous silicon layer must be carefully controlled so that programming is accurately performed.
Many configurations are possible for the antifuse. One configuration is the vertical antifuse is frequently preferred because it establishes an electrically conducting path through a via or window. See, for example, U.S. Pat. No. 5,100,827, issued on Mar. 31, 1992 to Steven A. Lytle, for a description of a vertical antifuse. The vertical antifuse is frequently preferred because it requires less area than does a horizontal antifuse. One configuration of the vertical antifuse has an aluminum interconnect covered by a dielectric layer. This dielectric layer is patterned to form vias which expose portions of the interconnect, and then layers of barrier material, amorphous silicon and aluminum are deposited. This configuration works well for many uses, but consideration of its structure shows that a via must be etched for each aluminum interconnect.
It would be desirable to effectively decrease the surface area needed by the circuit by reducing the numbers of vias that must be used per interconnect.